Scanners convert hard copy analog images on a media such as a transparent film, into corresponding electrical image signals (typically digital image signals). The use of scanners has become widespread for a variety of applications, including storing, manipulating, transmitting and displaying or printing copies of the images. For example, images captured in photographic media are being converted to digital data and stored on compact discs for readout and display as a video image or for printing with various types of color printers. In order to capture the photographic image digitally, an image frame is typically scanned with a light beam or line, and the light transmitted through the image is detected, typically as three primary color light intensity signals, and digitized. The digitized values may be formatted to a standard for video display and stored on optical or magnetic media. Such scanners (sometimes referenced as "film digitizers") take a variety of forms and the various common aspects of film digitizing, particularly line illumination and linear CCD-based digitizers, are described in greater detail in U.S. Pat. No. 5,012,346. The region through which a film is passed for scanning is often referenced as a film gate.
In scanning film, it is imperative to drive the film at an extremely constant velocity in addition to providing film flatness at the scan area (that is, the area from which light from the light source passes through the film and is received by the sensor). Failure to drive the film at a constant velocity (such as might result from film slippage) can result in an actual line of the image reaching the sensor, being considered by the digital electronics processing to be an image at a different location on the film. The digital image corresponding to the scanned image, will therefore be distorted. On the other hand, failure to maintain film flatness at the area or line being scanned, results in some parts of the scanned area or line being in focus on the sensor, while other parts are not. Thus, the resulting digital image will contain portions which are out of focus.
Rotary film gates have been developed as described in U.S. Pat. No. 5,088,813 and U.S. Pat. No. 5,153,733 which can alleviate the above problems. Those patents disclose film gates which include two concentric discs which support the film at edge margins while guiding it through the gate. Two elongated belts hold the film in position adjacent an arc of the discs. The curvature forced on the film as it passes through the rotary film gate causes it to be flat in a line across it at which scanning takes place. The described scanner of U.S. Pat. No. 5,153,735 drives the two discs directly with a servo motor. However, with this arrangement the size of the motor must be relatively large in order to provide the necessary torque. Use of a servo motor also requires an encoder to provide accurate feedback information on the motor velocity. Even if the servo motor was replaced with a stepper motor, a large motor would still be needed for the necessary torque. Further, since from a given stepper motor and associated drive circuitry only a limited number of steps per revolution can be obtained, such a direct drive arrangement would only allow relatively large stepped movement of a film through the gate and hence only relatively low scanning resolution could be obtained.
The rotary gate described in U.S. Pat. No. 5,088,813 can alleviate the foregoing problem, since the two discs which support the film are not driven directly by the motor, but are caused to rotate by a motor rotating a shaft of smaller diameter than the two discs through a pair of belts. The device described in U.S. Pat. No. 5,088,813 uses two non-stretch belts. Such non-stretch belts transmit the exact velocity of the motor to the discs without variations occurring due to belt stretch. However, because such belts are non-stretch, in order to maintain the necessary contact between them and the discs it is typically necessary in practice to provide a belt tensioning mechanism which can be loosened for belt removal or installation, and tightened for operation. Furthermore, rigid belts need to be matched with fairly high precision, and run on precision idlers with close size and location tolerances. All of the foregoing considerations add to the cost of constructing a scanner using rotary gates.
It would be desirable then to have a film scanner with a rotary gate or similar film gate, which is relatively simple to construct, does not require high precision in component location, and on which the belts can be readily installed during assembly of the scanner and during field replacement.